The present invention relates to positional calibration of servo-motor driven systems; and more particularly to an integral, continuous calibration system of the position sensor circuit of a galvanometer.
Galvanometer scanners are often used, either singly or in multiples, to point a light beam with high resolution, linearity, and repeatability. As an illustrative example of a demanding application, a pair of galvanometers arranged in Cartesian coordinates cooperate to point a laser beam over a solid angle of 30 degrees to a precision of 1 micro-radian or less anywhere in that field of view. The accomplishment of this task requires that the system be carefully calibrated in advance, to remove all the geometrical errors in mounting of the parts, and correct all of the residual non-linearity in the position detectors used to provide the feedback error signals to the servo system.
This calibrating is often done by commanding a series of positions in the field of view, recording the actual positions achieved, measuring the actual positions, and generating a set of correction factors by desired field position which are then combined with the command signal in such a way that the final position corresponds to the intended command. This set of correction factors is often stored in a look-up table. Such a table is constructed with rows and columns of cells, each representing a solid angle position. In each cell is stored one or a pair of correction values to be utilized when a measured position coincides with the cell""s field position.
Due to the complexity of the overall system of which the galvanometer scanners are but a part, and because the task for which the system is designed is usually a repetitive production task, such as drilling 1000 via holes per second in pre-wired boards or printed circuit boards, it is desirable for operating cost considerations that the system, once calibrated, operate continuously around the clock for extended periods without the need for periodic time consuming maintenance or adjustment.
Since a two dimensional, 30 degree solid angle field of view contains approximately 2.5xc3x9710{circumflex over ( )}11 resolvable points, the calibration process is complex and very time consuming. The calibration is more accurate when a larger number of points are utilized, but the increased number of points equates to more time and expense. Although by necessity carried out with the aid of high-speed data processing equipment, the calibration process usually takes several hours to complete. The calibration process is lengthy and tedious, but provides an accurate means of ensuring that the actual position is the same as the commanded positionxe2x80x94at least as of the time the angular position was calibrated. For angles between the calibrated points simple interpolation is used.
Unfortunately, the galvanometer scanners are inherently incapable of maintaining the linearity and precision of their position detectors over long periods of time. Nor are they immune entirely to the influences of change in temperature and relative humidity in their operating environment. As a result, the galvanometer or galvanometers begin to drift away from their calibrated condition immediately after calibration, and eventually again produce errors in pointing that offends the limits of accuracy required of their operation. Because of the high-speed production of parts that is the purpose of the system, it is often the case that a considerable quantity of scrap has been produced before the out of tolerance condition is detected.
A number of calibration techniques have been used in the past to re-establish the angular relationship to account for the drifts. One such method, termed in-field fiducials, uses detectors positioned in the field of view with special locations defined by X and Y coordinates. The differences are translated into factors that are stored in the machine circuitry and used to recalibrate the system from time to time. Others use a sample product every hour to define error values by physical measurement, and plot the deterioration of performance. It is also possible to employ fences as thresholds to determine when to recalibrate.
But, in a production run on a system with a capital cost that may approach one million dollars, stopping production in order to calibrate a relatively cheap component is not cost-effective or desired. The production machines need to run continuously, day and night for seven days per week, in order to be efficient.
As stated, there are a number of ways to recalibrate the system. The in-field fiducials are not part of a galvanometer head but are part of the overall machine. The in-field fiducials are targets, light detectors that signal when illuminated. About once a minute the system makes a measurement of the target sensors. There is processing required to compute the error amount, which requires some computational time as well as computer resources. Direct position error of load is achieved by this method.
Although in-field fiducials can be designed and manufactured as part of a new system, it is very difficult to upgrade or convert an existing system after the fact, because of the high degree of precision in positioning the target sensors remotely from the galvanometer head. Even if the targets/fiducials are placed inside the galvanometer head and look at the back of mirrors, an alternate configuration that has been tried, this is an intermediate step and still requires stopping and running a separate procedure and taking processing time to compute the calibration factors. Finally, all this does is calibrate the load with respect to the head.
Besides the calibration to resolve individual galvanometer characteristics, there are latency issues. The latency issues arise because the acceleration and maximum speed of galvanometers are limited. For example, the time to go from point A to point B is a time T. But, the time to go from point A to point 2B, is not 2T. It is necessary to calibrate these motions so the time intervals of a large number of points are measured and interpolation is used for points in-between the measured points.
For illustrative purposes, suppose a command signal versus time, such as a step function position command that lasts for some arbitrary time, is injected into a galvanometer scanner system. The signal has infinite slope, which only occurs in ideal and not practical agreements. Because of inertia, the system can not respond instantlyxe2x80x94it accelerates as the command is applied. There is some latency because it takes time for the system to detect the command and the magnitude level. At some future time the position of the load sensor reaches the desired position.
In general, because of inertiaxe2x80x94similar to a mass spring systemxe2x80x94the system acts as a tortional spring on each end of the shaft with respect to the motor. The ideal situation seeks to minimize latency and settling, and the stiffer the system the more ideal the system performance.
The calibration or recalibration process is typically done by calibrating the galvanometer before or during a pause in the manufacturing process. It is necessary to build the head and set up the system to perform a point by point array in the field of view. An average sequence may start with 64 points consisting of corners and middle points, measuring these points by the various methods known in the art. The field of view may consist of 106 points, so when a particular point is commanded it is necessary to interpolate from the look-up table to obtain the corrected position. The number of resolvable points are much greater than the calibrated number of points and it is therefore necessary to interpolate from the look-up table to obtain the best fit gain and offset.
Gain and offset are the two components or factors that control where in the box or field of view the command is pointing. From initial calibration measurements, initial calibration data is converted into gain and offset components. A look-up table is generated in order to correct the commanded point from the point that was actually measured to the intended point, by providing the gain and offset correction to be added to the position directly measured by the position sensor for each axis. This type of system performs adequately in ideal operating conditions, but nonideal conditions significantly impact the position. In particular, temperature changes influence the performance of the galvanometer and skew the gain and offset values so that the calibrated position is no longer accurate. Temperature effects are continuous and discontinuous, and significantly effect galvanometer performance.
The errors generally accrue over time, while the system is operating, leading to inaccuracies that eventually become intolerable if not corrected. An error in system gain can be described as a change in distance along either or both axis between the actual positions and the positions as measured from the same two calibration position commands when repeated at a later time. If the two positions are initially characterized as having a particular value on each axis, an error in gain is proportional to the change in the sum of these values. An error in offset will be apparent as a change in the position of one or both actual positions resulting from a later repeat of the respective initial calibration position commands.
Based on testing, the general trajectory of the temperature effects is a drift at about a 45 degree angle, this angle suggesting a substantially equal amount of drift in each of two similar galvanometers of a two-dimensional scanner. But, the effects are discontinuous, having sudden motions where the effects of the gain and offset may actually reverse direction. Continued operation of the galvanometers has a rather large distribution of errors in position that is difficult to predict or calibrate. These errors cannot be tolerated for those operations that require a high threshold of accuracy.
In summary, galvanometer recalibration is a necessity, and the only remedy to date has been to stop production and re-calibrate. The cost of the scrap, the cost of down-time of production machinery, and the cost of re-calibration are significant factors for this kind of manufacturing. The designers of galvanometers have been searching for a way or ways to minimize both the frequency of re-calibration required and the amount of scrap produced before the need for re-calibration has been discovered.
Accordingly, it is an object of the present invention to provide a non-contact means of continuously re-calibrating a galvanometer while in use.
Another object is to provide for continuous optical recalibration of the full range position sensor circuit and lookup table of a galvanometer, by using optical sensors illuminated through collimating slits in a calibration rotor closely coupled to the load so as to indicate at all times during the operation of the galvanometer the precise moments when the load is present at one or more known, unvarying fiducial positions within its normal angular range, and comparing the known fiducial position to the concurrently reported position from the position sensor circuit and lookup table, then calculating errors in the reported position and providing updates to the lookup table so as to regain the initial calibration accuracy.
Yet another object of the invention is to provide a two axis scanning device with a non-contact, continuous calibration capability in each axis that functions without interruption to the ongoing operation of the scanning device.
Still another object is to provide a dual axis electronic module that controls two specially constructed galvanometers using inter-module command, position, and temperature control interfaces. Each galvanometer has a full range capacitive position detector (PD) and an optical reference system. It is well known in the art that the PD exhibits error over time due to mechanical and thermal conditions. The present invention utilizes an optical reference that is stable over time to recalibrate the galvanometer without stopping the system and without undue complexity of processing resources and time.
One of the fundamental concepts of the invention is to compare the instantaneous value of the corrected position feedback signal with a known reference or fiducial position value that represents the value that the feedback signal had when it was initially calibrated. Any difference between the current PD signal and the calibrated reference PD signal represents a PD error. Any PD error is noticed immediately and an appropriate change to the value in that cell of the look-up table can be generated. As a result, the drift of the galvanometer in each axis, at a pre-selected one or more values on the respective axis within its normal range of motion, is continuously monitored and corrected each time it reaches the pre-selected value, so that the PD subsystem never goes out of calibration unless the system drifts out of the range of error correction capability.
The assumption behind the algorithm of the present invention is that the reference signal is perfectxe2x80x94and since this process takes place on the fly, that the position feedback signal is sampled at exactly the moment that the reference angular position is represented by the calibrated value from the look-up table. In practice, assuring that these conditions are met is extremely difficult and represents one aspect of the inventive subject matter of the present invention.
In theory, the system would stop at an exact location and allow a measurement of that exact location. However, this is not practical. First, no mechanical system of hard reference stops is practical because the degree of precision required is a fraction of a micron. Even if a sufficiently stiff structure could be constructed, wear on the contacting parts would quickly render the stops unreliable in angular position.
In addition, the process of finding these stops, which must of necessity be outside the operating range of the galvanometer, requires that the production process be stopped periodically so that the stops can be located. And, the time required for this type of calibration is the sort of costly cessation of production that the invention is intended to eliminate.
Of the non-contact methods known, capacitive sensors are themselves notorious for instability. Magnetic sensors of the Hall effect type are both unstable and susceptible to the magnetic fields generated inside the galvanometer during its operation. However, optical sensors are readily available which are both stable over long periods of time, and relatively insensitive to magnetic fields, temperature, and humidity changes.
In general, systems incorporating PD are adequate if the calibration of the angular relations of the position sensor and the load is quantified and maintained. But, as alluded in the background section, in the real world, as the system ages and as temperature changes, the angular relationship between the position sensor and the load changes. The changes brought on by mechanical and thermal conditions are well known in the art and the necessity to recalibrate and reestablish the angular relationship may be conducted once a day or once or an hour depending on the desired accuracy.
It is therefore a further object of the invention to provide a galvanometer calibration system where this inherent drift is self-correcting. The continuous, on-the-fly recalibration process produces a correction value that is applied directly to the look up table cells to re-calibrate the original values. Furthermore, the accuracy provided by the invention is maintained over long periods of operating time until the limits of the recalibration technique are reached and no longer can bring the system into conformity.
A yet further object is to perform correction on the fly and in real time, continuously during operation of the galvanometer. A still further object is to hold results for trend analysis and defer corrections until needed so no operating time is used up in the correction. A still yet further object permits altering of the reference angle or offset at which a calibration position is applied.
An additional object is to provide for correcting gain and offset, as well as for customizing limits of tolerance and changing limits on the fly.
Another additional object is to provide a means of alerting and signaling when an out of tolerance calibration condition arises. As is well known in the art, re-calibration prolongs the production life, but eventually the errors will require a more formal maintenance and mechanical adjustments to bring the system back into a working region. Such signaling means includes audible and visual alerts to bring attention to the production line to limit the amount of scrap produced.
One of the purposes of the present invention is to provide retrofits for existing systems to take advantage of the benefits of the present invention without purchasing an entirely new system.
The present invention significantly reduces the overall distribution of the position errors. A purpose of the system is to get from the initiation of a command to an adequately settled position of load in minimum time. The position of the load is what really matters, not the various sensors. But the sensors are needed to ensure the commanded position of the load equals the actual position of the load.
One goal of the invention is to provide for adding a new position sensor to the galvanometer shaft. This may be accomplished by putting an extension on the shaft and adding a low mass calibration reference rotor with two radially oriented reference position slits displaced around the rotor. The rotor is preferably proximate the load on the shaft, so that the reference rotor and load are rigidly interlocked, rotationally speaking. While the calibration sensor does not have much dynamic range, and may be configured for only two values in the arc or range of load rotation, it is extremely precise at these locations and uses all dynamic range within the two slits representing the two calibration points. The system may be designed to be extremely rigid and mechanically stable, and made insensitive to temperature variations as, for example, by using temperature insensitive materials such as invar.
It is yet another goal of the invention to have two galvanometers cooperatively configured at right angles to provide X and Y movement control of a light beam over a two dimension target area. Two calibration reference positions are selected on each axis of the target area, one on each side of the center or home position within the field of view, the values of which are defined after an initial calibration of the load angle with respect to fiducial positions in the target area.
Selecting just two positions with respect to the range of rotation of each galvanometer, which equates to two positions on each axis of motion on the target area, reduces processing resources and the time needed for processing when the positions are used for the calibration process of the invention. However, other schemes employing one or more positions for each axis of motion may be implemented by and are within the scope of the invention. Various embodiments of the invention also reduce hysterisis in the system and differentiate between gain and offset.
One of the principles of the present invention is a stable, load-locked reference system integral to the galvanometer that is used as a true position reference of the load. Any difference between the true position reference whenever it occurs, and the concurrently reported position by the full range position sensor and lookup table or equivalent initial calibration data, is interpreted as an error of the position sensor. Two rotational positions for each galvanometer, which equate to two values on each axis of motion in the target area, are employed to enable separation of gain from offset and to apply two multiplicative corrections to the interpolated look-up table command, thus providing corrections in both X and Y axis in a two dimensional scanning system.
With respect to the target area of a two galvanometer, two dimension system, the aspect of the invention for calibrating the position detector output may be explained as consisting of a smaller or inner view calibration frame or fence within the full field of view of the target area. For example, with a square target area with center 0,0. for each axis put two reference fiducials in the field of view, the four reference fiducials defining the inner view frame size and location. Although the percentages are not critical, offsets of the frame sides may be selected as 80% and 20% from center for each of the X and Y axis. The offset inner view frame is sized and located so that any movement of the system away from the home or center position, is likely to cross one side or another of the inner view frame.
At the moment of crossing a fiducial fence, or side of the position reference frame, the system takes a snapshot of the measured position indicated by the full range position sensor, as the direct measurement corrected by initial calibration data from the lookup table or however else applied. It then compares the pre-determined reference position of the respective fence to the full range position sensor""s measured value on the associated axis, and calculates and applies the required error correction to the correction lookup table or equivalent mechanism. All this occurs while the system is working on the fly, and does not impede productivity. This eliminates downtime from re-calibrations and extends time between system calibration.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only a preferred embodiment of the invention is described, simply by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invent on.